1. Field of the Invention
The present invention relates generally to overload protection circuits, but more particularly to a battery protection circuit for portable radios used in hazardous environments.
2. Description of the Prior Art
Overload protection devices, such as fuses, series current limiters, and electronic crowbar circuits, are well known and widely used in the art. Of these prior devices, few are applicable for use in hazardous environments--locations in which the atmosphere contains gas, vapor, or dust particles which could easily be ignited by sparks or thermal heating. A fuse, for example, poses a thermal ignition threat under overload conditions. Similarly, current limiting circuits generally allow a maximum load current to flow under fault conditions. Although the current shutback feature of a crowbar circuit is desirable, these devices exhibit a slow response time allowing the possibility that sparks will be produced.
Even fewer prior art overload protection circuits accommodate the specific problems of battery operated equipment such as portable two-way radios. Portable applications present the additional requirements of low power consumption during use; negligible power consumption when not in use; and minimal voltage drop across the battery protection circuit. Furthermore, portable applications introduce severe size and cost limitations such that the protection circuit can be built inside each individual battery housing.
Bogut, in U.S. Pat. No. 3,686,530, entitled "Current Limited Battery Pack," presents an example of a battery protection circuit incorporated into the battery housing. Bogut controls a series transistor or relay by sensing excessive current through a series resistor to fire a silicon-controlled rectifier (SCR). Unfortunately, equipment specifications for certain types of hazardous atmospheres require either continuous current limiting capability, or a faster response time than this technique is capable of producing. Furthermore, the Darlington transistor pair exhibits a substantial voltage drop.
A need, therefore, exists for an intrinsically safe battery protection circuit that meets the stringent specifications for use in explosive atmospheres while at the same time being compatible with the requirements of portable radio applications.